Attention is essential for all daily tasks, and failure to control attention can have tragic consequences. An understanding of the neuronal mechanisms that guide attention will be needed for any comprehensive treatment of attention disorders, and is moreover likely to provide important new insights into the mechanisms of sensory processing and perception. The visual system is an ideal place to study attention, not only because optimal visual performance is critical for so many activities, but also because our relatively advanced understanding of the functional organization of the visual system makes it uniquely suited for addressing cutting-edge issues. The experiments proposed here will advance our understanding of attention by identifying distinct components that make up visual spatial attention and characterizing how neurons in different structures in the brain contribute to those components. New data from our lab show that the behavioral enhancement associated with spatial attention depends on two separable mechanisms, which are related to changes in behavioral sensitivity and changes in behavioral criterion. Moreover, some attention-related changes in neuronal activity in visual cerebral cortex are specifically related to changes in sensitivity and not to changes in criterion. These results reveal that attention encapsulates multiple processes that have been conflated in previous work. The proposed experiments will build on these results to characterizing distinct mechanisms that contribute to attention and the neuronal mechanisms that support them. In doing so, they will greatly advance our understanding of precisely what attention is and how it relates to specific sensory and cognitive representations in the brain. The first specific aims it to identify which brain structures suppor attention-related changes in behavioral criterion, which are not encoded in the region of visual cortex we have studied. We will record attention-related neuronal modulations in three carefully selected brain structures: the lateral intraparietal area in visual cortex, the prefrontal cortex, nd the superior colliculus in the brainstem. These recordings will distinguish the contributions of major stations to different components of visual spatial attention. The second specific aim is to explore whether attention can be further divided into specific components. Attention is typically discussed in terms of selecting one stimulus or location over another, but attention also has a component related to intensity (high or low levels of attention). The intensive aspect of attention has been little studied and its neuronal underpinnings are poorly understood. We will separately manipulate the selective and intensive components of attention while recording the responses of individual neurons in visual cortex. The results will establish whether different brain structures contribute differentially to the selective and intensive aspects of attention.